Lens and Cataracts Flashcards
the lens is a ____ structure located within the ____
transparent, biconvex;
posterior chamber
the lens is suspended in the posterior chamber via ____
zonules (suspensory ligaments)
the lens focuses ____
light rays to a point source on the retina
the lens contributes ____ of total refractive power of the eye
13-17 D
the lens has a ____ refractive index (____)
high; 1.3-1.4
the lens is composed of ____ and is divided into ____
epithelial cells with a basement membrane;
the embryonic nucleus, fetal nucleus, adult nucleus, cortex, and capsule
during gestation, a single layer of _____ forms a hollow sphere, called the _____
epithelial cells; lens vesicle
_____ cells of the lens vesicle elongate to become primary lens fibers; they fill the lens vesicle and form the ____
posterior;
embryonic nucleus
_____ cells of the lens vesicle near the equator divide to produce new epithelial cells; these cells elongate to become secondary lens fibers and form the ____, which surrounds the ____
cell elongation continues until ____; form ____, allowing cells to remain tightly connected during accommodation
as cells reach the sutures, ____; cells consist of ____
anterior;
fetal nucleus;
embryonic nucleus
nasal and temporal cells meet (suture); interlocking ball-and-socket and tongue-and-groove junctions
nuclei and organelles are degraded; only a cell membrane, cytoskeleton, and crystallins (proteins)
the anterior suture appears as ____, while the posterior suture appears as ____
an upright Y;
an inverted Y
after birth, _____ cells continue to divide and elongate;
secondary lens fibers produced after birth form the ____, which surrounds the ____
newly produced secondary lens fibers are located in the ____
new fibers continue to surround older more central fibers; growth results in concentric layers of _____;
_____ cells are removed from the lens; cells in the center of the lens are ____
anterior;
adult nucleus;
fetal nucleus
cortex
secondary lens fibers;
no;
the oldest cells in the body
the basement membrane of the outer epithelial cells is called the _____; it surrounds ____
lens capsule;
the entire lens
the lens capsule is composed of _____; arrangement makes the lens capsule ____
primarily type IV collagen and GAGs;
highly elastic
____ lens capsule thickens throughout life; lens capsule components are secreted by _____; becomes the ____ BM in the body
anterior;
anterior epithelial cells;
thickest
____ lens capsule remains the same thickness throughout life; no _____ to secrete the lens capsule components
posterior;
posterior epithelial cells
the lens absorbs _____
ultraviolet light
_____ in the lens absorb most of UV-A (320-400 nm) radiation to prevent it from _____; absorption of UV radiation leads to production of ____
chromophores (yellow pigment);
reaching the retina;
free radicals –> DNA damage and protein aggregation
the lens refracts _____
visible light (400-700 nm)
refractive power of the lens changes with ____
contraction of the ciliary muscle
What happens with contraction of the ciliary muscle?
zonules relax –> lens thickness increases –> lens equatorial diameter decreases –> anterior lens radius of curvature decreases (anterior surface curvature steepens) –> lens refractive power increases (> 20D) = accommodation
with age, ability for the lens to change its curvature is ____; ciliary body moves progressively ____; lens _____; lens rigidity ____; nucleus becomes ____ than the cortex
impaired (presbyopia); inward toward the lens; thickens; increases; stiffer
the lens transmits ____
visible light (400-700 nm)
for optimal light transmission, the lens must be ____
transparent
the ____ are the only transparent tissues in the body
lens, cornea, vitreous
contributions to lens transparency
- lack of blood vessels and nerves
- orderly arrangement of lens fibers with uniform spacing between lens fibers; cells are hexagonal, allowing for tight, uniform packing of cells
- few cellular organelles in the visual axis
- high concentration of crystallins
- Na+/K+/ATPase in epithelial cell membranes
- presence of antioxidants glutathione and ascorbic acid (Vitamin C); neutralize free readicals
What are crystallins?
water-soluble proteins that comprise over 90% of total cellular protein; types alpha, beta, gamma;
alpha crystallins function as molecular chaperones - stabilize proteins that are partially unfolded and prevent them from aggregating;
beta and gamma crystallins have unknown functions
with age, the lens loses ____
transparency
with age, chromophore concentration ____;
leads to _____ and _____
increases;
yellowing of the lens;
DNA damage and protein aggregation
with age, glutathione concentration ____;
leads to _____
decreases;
DNA damage and protein aggregation
with age, alpha crystallin concentration ____;
by age 45, _____;
leads to ____
decreases;
no alpha crystallins evident in the lens nucleus;
protein aggregation
with age, epithelial cell membrane permeability ____; leading to ____
increases;
solutes and water enter epithelial cell –> swelling of lens fibers; disrupts orderly arrangement and uniform spacing of lens fibers
nuclear sclerosis
hardening and yellowing of the lens nucleus
nuclear sclerosis etiology
age-related
nuclear sclerosis demographics
everyone, typically begins around age 50
nuclear sclerosis laterality
bilateral > unilateral (can be asymmetric)
nuclear sclerosis symptoms
- asymptomatic
- blurred vision, decreased night vision
- poor color discrimination
nuclear sclerosis signs
- yellow lens nucleus, may progress to brown (brunescent)
- refractive shift, typically a myopic shift (“second sight”)
nuclear sclerosis management
- monitor: 6 month to yearly observation, SL exam, grade severity
- correct refractive error
- refer for cataract surgery when BCVA is 20/40 or worse (most states’ legal driving requirement) or when the cataract is impacting ADLs (vision can be better than 20/40)
nuclear sclerosis pearls
- NS can also be white or “milky”: greater effect on vision, larger myopic shift
- slowly progressive
- can progress to a mature or hypermature cataract
- historically, medical insurance would only pay for surgery when the vision was 20/40 or worse
cortical cataract
opacification of the lens cortex
cortical cataract etiology
age-related
cortical cataract demographics
everyone, typically begins around age 50
cortical cataract laterality
bilateral > unilateral (can be asymmetric)
cortical cataract symptoms
- asymptomatic
- blurred vision
- glare/night vision problems
cortical cataract signs
- cortical water clefts and vacuoles in the early stages
- spoke or wedge-like opacities in the cortex- begin in the periphery, coalesce (get thicker and larger) as the opacities encroach the visual axis
- refractive shift, typically a hyperopic shift, lenticular cyl
cortical cataract management
- monitor: 6 month to yearly observation, SL exam, grade severity
- correct refractive error
- refer for cataract surgery when BCVA is 20/40 or worse (most states’ legal driving requirement) or when the cataract is impacting ADLs (vision can be better than 20/40)
